We had a problem where a page would display OK in IE but if we tried to open it in Sharepoint Designer (SPD) for editing we had an error message. We also noticed that the size had grown from a few hundred Kb to over 10Mb. Turns out what had happened is the last time it was saved SPD had added thousands of 'blank' lines. Apparently this is a known bug but unresolved.
The solution is to open the file as a text file (which SPD will do) and remove all the blank lines. Note that few of the lines are truly blank (i.e., just a \n) but often have multiple tab characters in them so a blank line removal function probably will not work. We found the only way to remove them reliably was to do so manually.